The present invention relates generally to a method of making extruded resin articles, and a method of preparing such a mixture to be used in making extruded PVC pipe.
For many years the pipe industry has used plastic, particularly polyvinyl chloride, in making pipe for pressure service, sewer service, telephone service, as well as for other intended uses. A common and well developed way of making PVC pipe is by means of extrusion. Generally, this process includes preparing a blend or mixture including PVC in particulate or powdered form and various additives, such as extrusion stabilizers, processing aids, and lubricants, also in particulate or powdered form. This mixture is normally prepared at a location which is remote of the extruder and conveyed to the extruder via one or more storage points. A typical method of conveying this mixture is by means of air transfer. The mixture is ultimately deposited into the extruder where the PVC is worked by one or more extrusion screws to fuse into the final product, such as a pipe.
During the original development of extruded PVC pipe, it became apparent that a substantially pure polyvinyl chloride pipe having satisfactory physical characteristics for its intended use could be extruded with the only exception being the ultilization of very small amounts of PVC stabilizers, extrusion processing aids such as fusion promoters and lubricants, and substances to improve the physicals of the ultimately produced pipe. Because of these capabilities coupled with the fact that at that time PVC resin was relatively inexpensive, most extruded PVC pipes were substantially pure PVC with, of course, small amounts of the additives just recited.
In the recent past, as a result of the apparent oil shortage, the polyvinyl chloride resin as well as other resins started to increase in price and became less available. This trend has continued causing various pipe manufacturers to initiate development of means to "extend" the use of PVC in their pipes, i.e., to provide a pipe which is as satisfactory for its intended use as a substantially pure PVC pipe but one which includes less PVC in combination with a lesser expensive filler material. One known extender of PVC resin is calcium carbonate which heretofore has typically been initially loosely mixed with the PVC resin, in particulate form, and conveyed along with the PVC mixture from point to point and eventually deposited into the extruder.
In order to be compatible with the extrusion process and to minimize the production of extruded pipe with adverse physical characteristics, it has been found that the calcium carbonate or, for that matter, other materials used to extend the PVC must be very fine, actually much finer than the PVC particles. As a result, when the PVC mixture including the much finer calcium carbonate or other filler is air-conveyed from the point at which the mixture is initially prepared to various storage points and finally to the extruder, a number of specific problems arise due to the filler segregating out from the PVC mixture. First, much of the finer calcium carbonate particles form a dust cloud as a result of the turbulence caused by the air conveyance system creating a dust conditon in the air around the open junctures at, for example, the various storage locations or, for example, at the point where the mixture is deposited into the extruder. This, of course, is quite annoying to the operators in the direct vicinity and, further, results in an annoying maintenance problem. Second, this cloud of calcium carbonate tends to settle on the internal surfaces of, for example, a storage container causing the mixture being fed to the extruder to have less filler, i.e., calcium carbonate, than it should. After a fairly large build-up of this calcium carbonate within the container wall, it has been found to eventually break away, falling into the PVC mixture and thus causing too much calcium carbonate to be fed to the extruder along with the PVC and additives which, in turn, results in process problems such as, for example, loss of melt strength. In addition to these problems, the clouds of calcium carbonate tend to also adhere to vacuum filters within the storage containers. This requires the filters to be cleaned or changed at shorter intervals than would be the case if the mixture did not include the fine filler particles.
As will be discussed in more detail hereinafter, Applicants have discovered a way to combine the PVC or other resin comprising the major constituent of the mixture with calcium carbonate or other such PVC extender to be used in the mixture to substantially minimize, if not eliminate, the aforediscussed problems. Applicants' resultant mixture is one which includes, for example, PVC resin and calcium carbonate or other such extender in fine powder form. However, this mixture reduces substantially the dust problem, material segregation problem, and filter maintenance problem of the types discussed above.